ON THE ACTION OF AIR AND WATER UPON IRON, 249 
surface has been in metallic contact with zinc, or other more 
powerfully electro-positive metal, while immersed in a solvent, 
have its surface removed by the file to the depth of 3.th of an 
inch or more, and be then immersed alone in fresh or sea water, 
oxidation does not take place at all for a considerable time, and 
only forms at length in minute detached tubercles. 
211. Thus it appears that wrought iron, which has been for a 
length of time in contact with an electro-negative metal in pre- 
sence of a solvent, acquires an electro- positive polarity, while that 
which has been so circumstanced with an electro-positive metal, 
acquires an electro-negative polarity. The same phenomena do 
not present themselves with cast iron to the same extent, but yet 
are discernible. 
To this curious subject of electro-polarization, Dr. Andrews’s 
experiments on bismuth and platina supply analogous instances ; 
and an interesting paper on the same, as effecting copper and 
platina, has been much overlooked in the thirteenth volume of 
_ the Journal of the Royal Institution, p. 200. I introduce these 
two last experiments (209. 210) here for the purpose of remark- 
ing, that no piece of iron which has before been used for any such 
experiments as the present, should be used again for a different 
one. Before becoming aware of this, I was tormented with 
anomalous results, 
212. I stated in my previous report, that the views of Payen 
and Dumas, viz. that the cause of tubercular corrosion was a 
slightly alkaline reaction of the corroding water, seemed to me 
unnecessary to account for the phenomena. Iam now enabled 
to state, that the sole essential circumstance to tubercular corro- 
sion is want of homogeneity in the metal corroded, and that I 
have obtained the most marked tubercular corrosion of cast iron 
in pure distilled water, and in acidulous fluids. The corroding 
agent must, of course, be such as will not dissolve the oxide pro- 
duced ; and it must be admitted, that all other things being the 
Same, the presence of an alkali greatly exalts the tendency to 
tubercular deposition of oxide, which may, however, take place 
without it. 
Of Cast Iron in simple contact with Zinc immersed in Sea 
Water. 
213. Cast iron, perfectly free from initial rust, is perfectly 
preserved from oxidation in sea water by an equal surface of 
zinc. The latter is oxidated; but the oxide formed is not 
transferred to the surface of the iron, nor does it adhere to 
that of the zinc: it is washed away in a flocculent form, and 
is partly dissolved by the saline contents of the sea water. 
